Implantable medical devices such as cardioverter/defibrillators are commonly configured to treat cardiac arrhythmias by delivering high voltage energy pulses to cardiac tissue. Implantable defibrillators commonly deliver therapy by way of electrodes positioned within or near the heart of the patient. Such therapy includes defibrillation therapy, which utilizes a sudden, high energy pulse designed to shock the heart of the patient out of a cardiac arrhythmia if and when a cardiac arrhythmia occurs. Implantable defibrillators also commonly incorporate pacing therapy, which utilizes very low energy pulses designed to trigger cardiac contractions in lieu of an adequately frequent natural heart beat of the patient.
Implantable defibrillators commonly incorporate a power source, such as a battery, which provides operational power to the componentry of the defibrillator, including electronics which manage the function of the device, monitor the condition of the patient in which the device is implanted and deliver therapy to the patient. Many or most device functions operate effectively continually, such as sensing the cardiac condition of the patient, or frequently, such as cardiac pacing therapy delivery in certain patients, and thus account for steady, predictable and, usually, low-level drains on the battery capacity. Defibrillation therapy, by contrast, usually occurs very infrequently in most patients, commonly with months or years between defibrillation therapy deliveries, owing to the generally infrequent occurrence of arrhythmias which require treatment. As such, defibrillation therapy is, from a standpoint of battery management, a large, sudden, essentially random drain on the battery of the implantable defibrillator.
Because implantable defibrillators often provide life-sustaining therapy to the patients, it is essential to the well-being of the patient to understand how long the battery may be expected to last until the battery will be discharged to a point of being unable to provide reliable therapy. Hence, with an implantable medical device (IMD), it is necessary to provide an indication prior to battery depletion to enable the device to be replaced prior to loss of function of the IMD. This is commonly referred to as an elective replacement indicator (ERI) or a recommended replacement time (RRT). One method used to set an RRT threshold is with the use of a time based algorithm that is started at the time of the implant of the IMD. This time based algorithm type of RRT system is adequate when the battery used in the IMD has a relatively large capacity and its performance is predictable. However, as IMDs shrink in size, the batteries used in the IMDs also need to shrink in size. Smaller batteries tend to have reduced capacity. Moreover, the performance of these smaller size batteries can vary broadly in both voltage performance and impedance performance. Because of these characteristics, the use of a time based algorithm may be unreliable for a smaller battery.
For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an effective and efficient method and system to determine an RRT of a battery.